Piezoelectricity or, synonymously, the piezoelectric effect was discovered by Pierre and Jacques Curie in 1880. The effect is manifested by the appearance of an electric potential across the faces of some materials when those materials are placed under pressure. Conversely, when a piezoelectric material (PEM) is subjected to an electric field, physical stresses are created in the material and it distorts, a phenomenon known as the converse piezoelectric effect.
Among the materials exhibiting the piezoelectric effect are crystalline substances whose unit crystal structure lacks a center of symmetry and polycrystalline substances which have been placed in a polarized state, called piezoelectric ceramics.
An example of a PEM is the piezoelectric ceramic PZT, an amalgam of lead, zirconium and titanium. Other PEMs include, without limitation, piezoelectric polymers and polymer-ceramic composites.
PEMs have many diverse uses. For example and without limitation, PEMs are used in thin film capacitors, non-volatile ferroelectric semi-conductor memory, optical wave guides, optical memory and display, SAW (surface acoustic wave) devices, medical ultrasound applications, gas ignitors, displacement tranducers, accelerometers, transformers, impact printer heads and ink-jet printer heads.
The piezoelectric effect involves a complex interaction of electrical and mechanical properties of a material; up to 18 piezoelectric coefficients may be necessary to fully quantify piezoelectricity in a particular PEM. Yet, in many of the above applications, it is necessary that the piezoelectric effect be relatively homogeneous throughout the volume of the PEM being employed to be effective in that particular application; thus, some level of quantification is required. To accomplish this quantification, the usual source of methodology is the IEEE (Institute of Electrical and Electronic Engineers) Standard on Piezoelectricity, IEEE Std. No 176-1987 (incorporated by reference as if fully set forth herein) which contains test methods for determining various piezoelectric constants in a given PEM.
The present IEEE standard, however, does not adequately characterize piezoelectricity for certain applications. An example, without limitation, of such an application is the drop-on-demand ink-jet printer head. In this application, an electrical signal applied to a piezoelectric ceramic wafer causes deformation in the wafer resulting in expulsion of ink from an ink chamber formed within the wafer. Recently developed drop-on-demand ink-jet printer heads may contain as many as 128 such ink chambers closely packed into a single piezoelectric ceramic wafer, each chamber being capable of being individually deformed by application of a localized electric field. If the amount of deformation of the PEM in the immediate vicinity of each of the 128 ink chambers is not substantially the same as that in the immediate vicinity of each of the other ink chambers, the amount and velocity of ink delivered from each ink chamber will not be the same and unacceptable variations in the resultant print will result. The situation is exacerbated when the ink-jet printer is a color printer. In a color printer, the amount and placement of ink expulsed from the ink chambers is directly related to the ultimate color of the image obtained; minute variations in the amount of ink and placement of drops ejected from each chamber can lead to poor color image reproduction.
To avoid the preceding problem, it would be desirable to be able to determine the uniformity of the piezoelectric effect throughout the volume of the piezoelectric ceramic wafer from which a drop-on-demand ink-jet print head is to be manufactured to assure that an electric field applied in the vicinity of each of the closely packed ink chambers within the wafer will produce the same amount of deformation in that chamber as in each of the other chambers thereby assuring a uniform delivery ink. The IEEE methods, however, afford only average values of the piezoelectic constants in a PEM, leaving open the possibility of localized differences which might, in the case of the ink jet printer head, for example, result in inconsistency in the amount of ink ejected from the individual ink chambers and thus poor print quality and/or color reproduction.
Thus, there is a need for a method for evaluating the uniformity of the piezoelectric effect throughout a PEM to determine whether it is sufficiently uniform for the intended end use of the PEM. The present invention provides such a method.